The present invention relates to kinescope driver circuitry. Specifically, the present invention operates in a projection television receiver, to reduce RF interference generated in high speed digital circuitry, used for generating an auxiliary video signal, from being coupled to the antenna input circuitry due to relatively long wire leads between the auxiliary video signal generator and the kinescope driver circuitry.
US-A-5 337 090 discloses a video output circuit comprising a bridged-T-type circuit and an inductor connected in series. The bridged-T-type circuit, the inductor and a plurality of stray capacitances constitute a low pass filter.
US-A-5 036 260 discloses a kinescope driver with a self biasing protection circuit that decouples the cathode of the CRT from a supply terminal in case of a fault condition.
EP-A-0 680 226 discloses a kinescope driver that includes a gamma correction. For achieving this feature, an output signal corresponding to the CRT beam current is fed back to the driver via a filter network.
Each of the above documents discloses, as part of a driver circuit, a pair of transistors in a cascode arrangement, for driving a buffer amplifier which, according to the latter two documents, comprises a class B transistor stage.
An auxiliary video signal generator in current television receivers may include an on-screen display circuit (OSD) for displaying information useful to a viewer, such as channel number and/or time, on the screen. In addition, an auxiliary video signal generator may also be used for displaying patterns useful for adjusting and calibrating the data stored in a digital convergence integrated circuit (DCIC), such as vertical and/or horizontal lines, dots, or color bars.
An OSD generator, for example, may be provided in a receiver to generate a video signal representing the image of the OSD. The OSD image representative signal is generally coupled directly to the final driver of the kinescope, bypassing the channel related television signal processing circuitry. The OSD image representative signal may be applied, alone, or may be time-multiplexed with the video component signal derived from the received television signal and the resulting video signal may be coupled to the cathode electrode of the kinescope.
As is well known, the OSD image representative signal is generated by digital circuitry in an OSD generator. Such an OSD generator is responsive to a high frequency clock signal, and includes digital circuitry switching at that clock frequency. Signals generated by such circuitry can carry substantial harmonic content well into the frequency range to which the tuner circuitry is sensitive. In a standard television receiver, such harmonics are attenuated by placing the tuner and its associated circuitry in a metal shield. Input and output terminals of the tuner are isolated by low pass filters where they pass through the shield.
In a projection television receiver, however, the enclosure containing the tuner and the auxiliary video signal generator used for the DCIC is physically separated from the enclosure containing the three kinescopes which generate the images projected on the passive display screen in a known manner. The conductor wire carrying, for example, the image representative auxiliary video signal to the kinescopes, therefore, is relatively long: e.g. several feet long. This wire acts as a transmitting antenna, and the relatively high frequency harmonics of the signal carried by this wire are transmitted back to the antenna input terminal of the tuner. The frequency of the radio frequency RF energy radiated by the harmonics in the image representative signal is in the range to interfere with the primary television signal.
It is possible to minimize RF interference using LC filters. However filters having the characteristics necessary to pass the portion of the image representative signal sufficient to produce a display of reasonable quality, and simultaneously attenuate the RF interference sufficiently, is a relatively complex filter, and requires substantial numbers of components and is expensive to fabricate and assemble. Apparatus for attenuating the RF interference without degrading the image which is relatively inexpensive is desirable.
A typical kinescope driver includes a pair of transistors coupled in a cascode configuration (see the aforementioned documents). One of the transistors, referred to as the lower transistor, acts as an amplifier for converting the voltage of the video signal to a current. The other transistor, referred to as the upper transistor, is coupled in a common base configuration to the cathode of the kinescope to isolate the video signal component of the cathode voltage of the kinescope from the collector of the lower transistor.
When applying the auxiliary video signal to the cathode of the tube, it may be desirable to avoid having a long wire conductor between the collector of the corresponding lower transistor and the emitter of the upper transistor in the signal path of the auxiliary video signal. This is so because the signal developed in such a long wire may, undesirably, act with the Miller capacitance, between the base and collector of the lower transistor, to amplify the aforementioned interference producing clock signal related harmonics.
It is desirable to reduce interference from oscillations on connection wires.
In accordance with the invention, which is defined in claim 1, a further transistor is interposed between the lower and upper transistors. The further transistor is coupled in a cascode configuration with respect to the lower transistor. The conductor wire coupling the collector of the lower transistor to the emitter of the further transistor is, advantageously, short. On the other hand, the wire coupling the collector of the further transistor to the emitter of the upper transistor may be long. Thus, the further transistor isolates the amplifying, lower transistor from the long wire. Therefore, advantageously, the signal developed in such long wire can no longer act with the aforementioned Miller capacitance of the lower transistor. Thereby, the RF interference is, advantageously, reduced.
In the drawing:
In the FIGURE, a television receiver front end (not shown) produces a video component signal in response to a received television signal, in a known manner. Such a receiver front end includes an antenna and/or cable input terminal, RF amplifiers, IF amplifiers, a tuner, and audio and video signal processing circuitry of known design. The video processing circuitry produces a video signal representing the image included in the television signal, or one video signal representing each color component making up that image.
The illustrated embodiment of the present invention is in a projection television system in which each of the color components (red, green and blue) is coupled to a separate kinescope. In the FIGURE, circuitry supplying a color component image signal to only one of the three kinescopes is illustrated. One skilled in the art will understand that each such kinescope has similar circuitry coupled to it, and will understand what portion of the illustrated circuitry is shared in common amongst all the kinescopes, and what portion of the circuitry is provided separately for each kinescope. One skilled in the art will also understand that the illustrated embodiment may also be used in a standard television receiver in which a single kinescope includes three electron guns, one for each of the color components, or a single electron gun shared by the three colors.
In the FIGURE, a video signal from the receiver front end (not shown) is coupled to a base electrode of a video amplifying transistor T1. An emitter electrode of the video amplifying transistor T1 is coupled to a source of reference potential (ground) through an emitter resistor R1. A biasing circuit (not shown) may be coupled to the base electrode of the video amplifying transistor T1 in a known manner.
An auxiliary video signal generator 10 such as, for example, an on-screen display (OSD) generator produces an image representative signal at an output terminal 10a. Generator 10 may be an auxiliary video signal generator such as, for example, a pattern generator used for calibrating a digital convergence integrated circuit (DCIC). Generator 10 receives a relatively high frequency clock signal (CLK). In the illustrated embodiment, the clock signal has a frequency of 8.56 MHz. Generator 10 includes digital circuitry, not shown in details, which is clocked at the clock signal frequency. The circuitry possibly includes a processor which may be responsive to viewer input, and generates a signal representing the auxiliary image, as described above. The output terminal 10a of generator 10 is coupled to a base electrode of an amplifying transistor T2 through a first biasing resistor R2. A second biasing resistor R3 is coupled between the output terminal of generator 10 and ground. An emitter electrode of amplifying transistor T2 is coupled to ground through an emitter resistor R4. Transistor T2 converts the voltage of the video signal developed at output terminal 10a of generator 10 to a collector current of transistor T2.
In carrying out an inventive feature, a collector electrode of the amplifying transistor T2 is coupled to an emitter electrode of a first, cascode, transistor T3 operated in common base mode. A source of a biasing potential is coupled to the base electrode of the first transistor T3. In the illustrated embodiment, the biasing potential is 3 volts. In addition, an RF bypass capacitor C1 is coupled between the base electrode of first transistor T3 and ground. The collector electrode of first transistor T3 is coupled to a collector electrode of the video amplifying transistor T1 through a collector resistor R5.
The collector of the video amplifying transistor T1 is coupled to an emitter electrode of a second, main cascode, transistor T4, operated in common base mode, through a series connection of respective resistors R6 and R7. A blanking circuit 20 of known design monitors the vertical and horizontal scanning of the deflection coils (not shown) associated with the kinescopes in the projection television in a known manner. An output terminal of the blanking circuit 20 is coupled to the junction of the resistor R7 and the resistor R6. A source of a main cascode biasing voltage is coupled to a base electrode of the second, main cascode, transistor T4. In the illustrated embodiment, the main cascode biasing voltage is 10 volts. An AC filter capacitor C2 is coupled between the base electrode of the second, main cascode, transistor T4 and ground.
A collector electrode of the second transistor T4 is coupled to a source of an operating potential through a series connection of a peaking coil L1, and a load resistor R8. In the illustrated embodiment, the operating potential is 225 volts. A buffer amplifier 30 has an input terminal coupled to resistor R8. In the illustrated embodiment, the buffer amplifier 30 may be a push-pull (class B) amplifier having series connected complementary transistors, not shown. The output terminal of the buffer amplifier 30 is coupled to a control electrode of a kinescope 40. In the illustrated embodiment, the output terminal of the buffer amplifier 30 is coupled to a cathode electrode of an electron gun in the kinescope 40.
In operation, the video signal from the receiver front end (not shown) is coupled to the kinescope 40 by a cascode amplifier formed from the video amplifying transistor T1 and the second, main cascode, transistor T4, as in prior art arrangements. It may be undesirable to couple the auxiliary video signal from the generator 10 to the kinescope 40 by coupling the collector electrode of amplifying transistor T2 directly to the emitter electrode of the second, main cascode, transistor T4 through a long wire conductor. This is so because the high frequency harmonics in the auxiliary video signal, resulting from the digital circuitry in generator 10, as described above, could have been significantly amplified by the operation of transistor T2. Amplification could occur because of the interaction of the impedance of such long wire with a Miller capacitance CT2, between the collector and base of transistor T2. Had the connection between the collector of amplifying transistor T2 and the second, main cascode, transistor T4 been made directly through a relatively long wire that could be several feet long, and without interposing transistor T3, the high frequency harmonics developed on this long wire could have been amplified and transmitted to the tuner input terminal of the receiver front end, and substantially degraded the operation of the tuner.
In carrying out an inventive feature, the first transistor T3, is coupled between the amplifying transistor T2 and the second transistor T4 through a short wire conductor connection, referred to as SHORT WIRE, such as less than one inch. Transistor T3 is arranged to isolate the collector of transistor T2 from the signal developed in the long wire connection between transistors T3 and T4 that could be several feet long. The relatively high frequency harmonics cannot develop because of the low impedance formed at the collector of transistor T2. The low impedance is obtained because of the short wire connection, the low emitter-base impedance and the usage of RF bypass capacitor C1. These harmonics are shunted to ground through the low impedance before being passed to the long wire that is coupled to the second transistor T4. Therefore, the signal developed in the impedance of that long wire cannot interact with Miller capacitance CT2 of transistor T2. Consequently, tuner operation is not adversely affected by any pick up of these harmonics. In addition, the first transistor T3 and the RF bypass capacitor C1 do not perceptibly degrade the image. Advantageously, this solution requires only one additional transistor T3 and one RF bypass capacitor C1, which are relatively inexpensive components.